US2013021658 discloses a head mounted display device that is designed to superimpose a computer generated display image on a background formed by part of the view of the surrounding environment. Such a head mounted display device comprises a display panel for displaying the computer generated image and an optical combiner that combines light of the display image from the display and background light along a same viewing axis to the user's eye, coming from the environment outside the head mounted device. The optical combiner combines light in a combiner area that forms only part of the field of view, so that the background light from the environment is visible adjacent the combiner area, without being combined with the overlaid computer-generated image outside the combiner area. The head mounted display device may be configured in the form of eyeglasses. Part of the system may be in integrated in the legs of the eyeglasses by which the eyeglasses are hooked to the user's ears. Google glass is an example of such a head mounted display device.
It has been found that overlay with the background can impair legibility of the display information. This could be solved by showing only the display image in part of the field of view without overlay, not as see-through, i.e. as an occluding HMD. However, an occluding HMD detracts from the view of the surroundings and it may affect stereoscopic viewing, especially if the display information if provided only to one eye. Maintaining visibility of the background overlaid with the display image has been found to improve visual comfort. However, the overlay still forms a burden for the user that wears this type of head mounted display device, because the user need more time to read the computer generated image. This increased reading time is an additional distraction from the surrounding environment.
US2014055324 discloses attenuation and more in particular blocking of background light behind the displayed information based on the state of the surroundings, e.g. it brightness. US2014055324 describes of an LCD to block the light by scattering the light. As is well known from use in displays, a liquid crystals can be used to block transmission of light by scattering it back.